A known advantage of a three-step inverter compared to a two-step inverter is that the three-step inverter has available three direct voltage potentials to simulate approximately sinusoidal voltages at the inverter output. Thus, the phases of a three-step inverter are supplied both by the positive or negative potential, of a direct voltage source and by a central potential, preferably corresponding to the zero potential. At the input of the three-step inverter, this central potential connection is made available as a "floating zero point" at the interconnection point of two intermediate circuit capacitors fed by the direct voltage source. It is also possible to form the three direct voltage potential by an alternating current transformer set up accordingly with downstream inverter configurations for the positive and negative potentials relative to the central potential.
In the inverter of FIG. 1, to form approximately sinusoidal voltages at outputs R, S, and T of the phases PR, PS, PT, the three direct voltage potentials U.sub.D+, U.sub.D-, MP are coupled through particular circuit elements in each phase of the three-step inverter for specific amounts of time at the respective output. In the phase PR, illustrated as an example in FIG. 1, these particular circuit elements are shown as T1 to T4, which are provided with antiparallel recovery diodes and arranged in series between the positive potential U.sub.D+ and the negative potential U.sub.D-. The interconnection points between T1 and T2 or T3 and T4 are coupled in a series arrangement through two coupling diodes and the central potential connection MP of the direct voltage source.
When three-phase machines are supplied with frequency controllable converters, it is advantageous to represent the electrical variables, especially the voltages and currents generated by the converter in the electrical machine, in the form of space vectors in a rectangular .alpha.,.beta.-coordinate system relating to the stator of the electrical machine. The publication "Raumzeiger Modulation bei Frequenzumrichtern" (The Modulation of Space Vectors with Frequency Converters), Antriebstechnik (Drive Engineering) 27 (1988) No. 4, pp. 38 to 42, briefly describes this method of space vector representation and the "space vector modulation" resulting therefrom. The space vector modulation is a result of a known subharmonic process used in inverter operation. The publication describes the method of space vector representation using the example of an asynchronous machine fed by a two-step inverter.
Due to the limited number of possible circuit state combinations of the valves in the inverter, the voltage space vector can initially only occupy discrete positions in the .alpha.,.beta.-coordinate system. Such a coordinate system is shown as an example in FIG. 2. The peaks of the discrete, permissible positions of the voltage space vector U*, which can be generated by a three-step inverter, are denoted with rectangles and marked with the numbers 1 to 27. The circuit state of the three-step inverter required to generate &he respective positions is represented graphically inside these rectangles. Inside each rectangle, three switches are represented. The switches symbolize the phases cf a three-step inverter and each can represent three possible circuit states.
For example, if a phase switch points upwards, then the potential U.sub.D+ is connected to the output R by bringing the valves T1 and T2 into circuit in the phase PR. If, in comparison, the phase switch lies horizontally, then by bringing the valves T2 and T3 into circuit, the central potential MP is connected to the output R. Finally, if such a phase switch points downwards, then, for example, by bringing valves T3 and T4 into circuit, the potential U.sub.D- is connected to the respective phase output.
To generate a voltage space vector with switch settings as shown in position 1, the phase switch SP for the phase PR must point upwardly and both phase switches SS, ST for the phases PS, PT must point downwardly. In this case, the potential U.sub.D+ is coupled to the output R of the phase PR, and the potential U.sub.D- is coupled to the outputs S, T of the phases PS, PT. One can see from the representation in FIG. 2, that some circuit states are equivalent, that is they generate the same voltage space vector in the .alpha.,.beta.-coordinate system. Thus, for example, the circuit states 16 and 22 produce the same voltage space vector rotated 60.degree. compared to the .alpha.-coordinate axis.
It is often necessary to generate voltage space vectors which do not conform with the above described discrete positions and therefore lie inside of the equilateral triangles fixed by the discrete circuit states 1 to 27 in the .alpha.,.beta.-coordinate system. For this purpose, with the help o the pulse modulation process known as the subharmonic process, connections are made back and forth cyclically with a certain switching ratio between discrete space vector positions. This is also known as space vector modulation. Thus, to generate the space vector U*.sub.1 shown in FIG. 2, it is possible, for example, to couple back and forth cyclically between the circuit states 1, 9, 16, 22, 15, 21 and 14. As a result of superimposing discrete space vector positions in this manner any space vector intermediate position can be approximated in the time average.
From the publication "A novel approach to the generation and optimization of three level PWM wave forms", PESC '88 Record, IEEE, April 1988, pp. 1255 to 1262 (especially FIG. 5), a special form of space vector modulation for three step inverters is described, which is known as double modulation. A periodic waveform, corresponding to the known subharmonic process comprising linear parts, serves as a modulation signal, whose maxima and minima specify a preferably standardized upper and lower scanning limit. FIG. 3 shows, as an example, such a modulation signal MS, which is preferably triangular. Its extremes define the preferably standardized upper or lower scanning limit +1.0 or -1.0, shown as broken lines.
In accordance with double modulation, to form the switching pulses for the valves in the phases of the three-step inverter, two separate, cophasal setpoint signal sets, which are not phase displaced relative to each other, are scanned by the modulation signal. An example of two such three-phased setpoint signal sets are shown in FIG. 3. They are designated as first and second setpoint signal sets and each set comprises three sinusoidal phase signal waveforms, electrically phase shifted from each other by 120.degree., U*.sub.RO, U*.sub.SO, U*.sub.TO and U*.sub.RU, U*.sub.SU, U*.sub.TU. Thus, corresponding phase signal waveforms in both setpoint signal sets, i.e., the waveforms U*.sub.RO and U*.sub.RU, are cophasal to each other. The phase signal waveforms of the first setpoint signal set lie, as a rule, in the upper range of the modulation signal, while the phase signal waveforms of the second setpoint signal set lie in the lower range. The value of the median line MLO of the phase signal waveforms of the first setpoint signal set is greater than or equal to the value of the median line MLU of the phase signal waveforms of the second setpoint signal set. In the example of FIG. 3, the phase signal waveforms of the first and second setpoint signal sets are provided with median lines MLO, MLU with the values +0.5, -0.5, and are scanned by a standardized modulation signal MS, which has a range of 2.
There is a need for a method to use the double modulation process to operate a three-step inverter, so that the electric variables at the output of the three-step inverter are provided with a harmonic spectrum that is favorable for an electrical machine that is supplied as a load.